The operating behavior of an internal combustion engine is critically dependent on the quality of the mixture preparation or carburation. Thus, the emission of harmful substances and the fuel consumption of an engine can be substantially reduced by a mixing of the fuel to the intake air that is adapted to the respective operating condition. This is especially true for a motor vehicle internal combustion engine equipped with a controlled 3-way catalyst. The catalyst serving the purpose of reducing the pollutant emission, however, operates with high efficiency only within an extremely narrow range of air ratios. In order to assure a maximum degree of conversion, the air/fuel ratio therefore must only deviate from a rated value representing the respective optimum by a few percent in every operating condition of the engine.
The probes on the basis of zirconium oxide that become ion-conductive at higher temperatures and that serve the purpose of determining the air ratio .lambda. respond comparatively slowly to changes of the partial oxygen pressure in the exhaust gas. They are therefore not suitable for cylinder-selective .lambda. control and shall be advantageously replaced in the future by metal oxide sensors whose response time amounts to only a few milliseconds. These gas sensors manufactured in planar technology usually have a base member composed of a ceramic material on whose surface two interdigital electrodes and a metal oxide layer (for example, SrTiW.sub.3, CeO.sub.2 or Ga.sub.2 O.sub.3) that conductively connects the electrodes are applied. A resistor element present at the back side of the base member allows the active heating of the gas sensor. When oxygen flows over the thermally activated metal oxide, then the resistance or, respectively, conductance thereof changes reversibly due to complicated adsorption process at the oxide surface. The oxygen concentration in the exhaust gas can therefore be determined in a simple way by a measurement of resistance or, respectively, conductance.
Deposits on the surface of the metal oxide layer which is only a few .mu.m thick can substantially influence the gas-sensitive and electrical properties thereof. Sensors on the basis of semiconductive metal oxides are therefore arranged in a housing in order to keep the particles present in the exhaust gas of an internal combustion engine away from the O.sub.2 -sensitive regions. The particles that deteriorate the sensor function derive from the additives and the contaminants of the fuels (lubricating oil, benzene, etc.) or, respectively, arise during operation of the engine due to friction. Typically, they have a size of approximately 1-2 .mu.m and, among other things, are composed of iron oxides.
U.S. Pat. No. 4,916,934 discloses an oxygen detector composed of a planar sensor element and of a cylindrical housing. In order to prevent a direct flow against the oxygen-sensitive layer, the sensor element and the gas entry apertures present in the generated surface of the housing are arranged in different planes. The baffles respectively allocated to the entry apertures are intended to intercept the particles present in the exhaust gas and to produce a circular gas flow in the inside of the housing.
The cylinder housing for a fast exhaust gas sensor disclosed by EP-A-0 503 295 likewise comprises a plurality of slot-shaped gas entry apertures whose edges overlap louver-like. Since the slots act as particle traps, deposits on the sensitive sensor layer are effectively suppressed.
In order to prevent deposits in the apparatus for detecting chlorine and chlorine-containing compounds disclosed by DE 35 00 088 A1, only test gases that contain no particles and liquid drops are supplied to the solid electrolyte that serves as sensor element. The largely non-porous solid electrolyte that is incorporated sealed in a housing separates two respectively cylindrical chambers, whereby the one chamber is filled with the gas to be measured and the other chamber is filled with a reference gas. Two respective housing bores that enable the gas exchange discharge into the chambers.